Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

What proofs are there for general relativity?

So far, GR has made the following specific predictions:

1...The entire orbit of Mercury rotates because of the curved geometry of space
near the sun. The amount of 'perihelion shift' each century was well known
at the time Einstein provided a complete explanation for it in 1915.

2...Light at every frequency can be bent in exactly the same way by gravity.
This was confirmed in the 1919 Solar Eclipse for optical light using
stars near the Sun's limb, and in 1969-1975 using radio emissions from
star-like quasars also seen near the limb of the Sun. The deflection of the
light was exactly as predicted by GR.

3...Clocks run slower in strong gravitational fields. This was confirmed by
Robert Pound and George Rebka at Harvard University in 1959, and by Robert
Vessot
in the 1960's and 70's using high-precession hydrogen maser clocks flown on
jet planes and on satellites.

4...Gravitational mass and inertial mass are identical.
Most recently in 1971, Vladimir Braginsky at Moskow University confirmed GRs
prediction of this to within 1 part in a trillion of the exact equality
required by GR.

5...Black holes exist. Although these objects have been suspected to exist
since they were first introduced to astronomers in the early 1970's, it is only
in 1992 that a critical acceptance threshold was crossed in the astronomical
community. It was then that Hubble Space Telescope observations revealed
monstrous, billion-sun black holes in the cores of nearby galaxies such as
Messier 87, Messier 33 and NGC 4261.

6...Gravity has its own form of radiation which can carry energy.
Russel Hulse and Joseph Taylor in 1975 discovered two pulsars
orbiting each other, and through careful monitoring of their precise
pulses during the next 20 years, confirmed that the system is loosing
energy at a rate within 1 percent of the prediction by GR based on the
emission of gravitational radiation.

7...A new force exists called 'gravito-magnetism'. Just as electric and magnetic
fields are linked together, according to GR, a spinning body produces a
magnetism-like force called gravitomagnetism. GR predicts that rotating bodies not only
bend space and time, but also make empty space spin. A NASA satellite called
Gravity Probe B will be launched in the next few years to see whether this
effect exists. This is a killer. If it is not found, GR is mortally wounded
despite its long string of other successes.

8...Space can stretch during the expansion of the universe. This was confirmed
by Edwin Hubble's detection of the recession of the galaxies ca 1929. More
recently in 1993, Astronomer Kenneth Kellerman confirmed that the angular
sizes of distant radio sources shrink to a minimum then increase at greater
distances exactly as expected for a dilating space. This is not predicted by
any other cosmological model that does not also include the dilation of space
as a real, physical phenomenon.